Neoplastic lymphoid cells of B-cell and T-cell type were maintained in long term transplant. The T cell lines showed a variety of Lyt 1, Lyt 2, and TL phenotypes, and an 8-fold range of terminal deoxynucleotidyl transferase. The B cell lines were Ig ion, Fc ion, usually Ia ion, and LyM ion. Anti Ia or anti LyM could inhibit Fc rosettes. Somatic hybridization of B lymphomas with drug-marker MPC-11. Hybrids showed greatly amplified synthesis and secretion of pentameric IgM. Somatic hybrids between MPC-11 and spleen cells from NZB x NZW F1 mice yielded hybrid clones, one of which secreted IgG3 antibody to ribosomal RNA, which did not bind to DNA.